Harmonic crystal oscillator



Nov. 20, 1951 B. H. SIMONS HARMONIC CRYSTAL OSCILLATOR Filed NOV. 10, 1949 I X,(0/P X ETC) 4 Sheets-Sheet l Bi w /Nl EN7'OR B. h. SIMONS .Q omwu ATTORNEY Nov. 20, 1951 B SIMONS 2,575,363

HARMONIC CRYSTAL OSCILLATOR Filed Nov. 10, 1949 4 Sheets-Sheet 2 FL/ /3 /3 c F/GZ/C 6 a =c7 OUTPUT lNl EN roe 5. hi SIMONS ATTORNEY Nov. 20, 1951 B. H. SIMONS HARMONIC CRYSTAL OSCILLATOR Filed NOV. 10, 1949 4 Sheets'Sheet f5 INVENTOR B. H. SIMONS AT TORNEY Nov. 20, 1951 B. H. SIMONS 2,575,363

HARMONIC CRYSTAL OSCILLATOR Filed Nov. 10, 1949 4 Sheets-Sheet 4 OUTPUT li /N l/E N TOR 5, H. S/MONS A r TORNEY Patented Nov. 20, 1951 UNITED STATES PATENT] OFFICE 2,575,363 HARMONIG CRYSTAL OSCILLATOR Burton H. Simons, Morristown, N. J assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 10, 1949, Serial No. 126,510

17 Claims. 1

This-invention relates to crystal-controlled oscillation generators, and particularly to harmonic mode crystal oscillators which may be utilized for producing radia frequency currents controlled by a mechanical harmonic or overtone mode fre-- quency of one or more associated piezoelectric crystal bodies working in a frequency spectrum which may be of the order of up to 50 megacycles or more per second.

One of the objects of this invention is to provide a harmonic crystal oscillator capable of producing oscillations at a mechanical harmonic mode frequency of one or more associated piezoelectric crystal bodies, without interference from the fundamental or lower mode frequency of such crystal body or bodies.

Another object of this invention is to improve the frequency stability of harmonic crystal oscillators.

Another object of this invention is to provide low current drain on the power supply source for harmonic crystal oscillators.

Another object of this invention is to reduce parasitic oscillations in harmonic crystal oscil lators.

Another object of this invention is to reduce the number of circuit components in oscillators operating at the different mechanical harmonic mode frequencies of a plurality of piezoelectric crystal elements.

A particular object of this invention is to provide means for preventing the associated piezoelectric crystal body or bodies from oscillating; at its undesired fundamental mode frequency and at the'same time permitting it to oscillate at its desired frequency or frequencies, such as the third, fifth or other mechanical harmonic mode frequency thereof.

Another particular object of this invention is toprovide a harmonic crystal oscillator circuit capable of operation over a selected range of harmonic mode frequencies corresponding to the several harmonic mode frequencies of a series or plurality of different frequency harmonic mode piezoelectric crystal bodies.

For precision control of the frequencyof an oscillation generator, it is desirable to utilize a piezoelectric crystal body because of its inherently high frequency stability, and where the fre-- quency desired is higherthan that which may .be-

and also electrical, resonance at harmonicsof; itsfundamentalthickness modefrequency, and accordingly may beexcited at any selected one of its mechanical harmonic mode frequencies by means of a suitable circuit; and in the case of the AT-cut or BT-cut quartz crystal plate referred to, the harmonic mode frequency may be nearly an exact third, fifth, seventh or other odd order overtone multiple of thefundamental mode frequency thereof and thereby provide av stable harmonic crystal body oscillating at its mechanical harmonic mode frequency for direct and accurate working frequency control purposes at a comparatively high crystal vibration frequency which may conveniently be avalue up to 5,0 megacycles per second, or a considerably higher value.

It will be understood that the harmonic mode vibration of the crystal element itself, rather than 'the fundamental mode vibration thereof, is of- In oscillatory piezoelectric crystal elements of the relatively high frequency type, it has been difficult heretofore to utilize in practical systems the higher order mechanical harmonics of crystal elements. In accordance with this invention, r

such mechanical harmonics of crystal elements Y.

" be controlled in frequency by the harmonic mode particular interesthere in orderto obtain directly a high frequency crystal control of the associatedcircuitfrequency. For this purpose, the mechane ical harmonic of the crystal element may be an odd order harmonic of the fundamental shear mode thicknessvibration in an AT-cut or BT-cutquartz crystal element, for example. Accordingly, with such a suitable type of harmonic crystal unit or units, the circuits provided in accordance with this invention make available an upwardly extended frequency range of reliable crystal-controlled operation.

In accordancewith this invention, a harmonic. crystal oscillator circuit is: provided which may frequency of the associated harmonic crystal body, orof a plurality of such crystal bodies working at points spaced at, intervals overa I 7 range of harmonic mode frequencies; and in acfrequency of the harmonic crystal body or bodies,

and at thesametime permit the circuit to oscillate only on the desired harmonic mode frequency of the crystal body or bodies.

The oscillator circuit may include a single electronic vacuum tube such as a grounded cathode pentode designed for high frequency use;and-

having the usual cathode, control grid, screen grid, suppressor grid and anode lor plate lec trodes, and provided with the usual power supply sources for heating the cathode heater and for supplying suitable positive"pote'ntials to; the creen grid and plate electrodes of the oscillator tube. Associated with the cathode, control grid and plate electrodes, which may be the oscillating n ator electrodes of the 'oscillat'or tube, a bridgecircuit network ormay be provided which may Lcomprise in an arm thereof "a suitable piezoelectric harmonic crystal unit; connected in the feedback path circuit between the plate and control gridelectrodes'of the oscillator tube.

ln a particular embodiment, the harmonic crystal oscillator V circuit provided in accordance with this] invention may" comprise a grounded cathodesingloscillatortube of the pentode type acting as a source of high gain, a frequency-dc termining bridge system which maybe connected in circuit relation with the plate, control grid and grounded jcathode oscillator electrodes of the pent'ode'type oscillator'tubeand which may .com prise a frequency-controlling harmonic crystal body disposed in one arm of the bridge and operatingfat its series resonance harmonic mode frequehcy in the plate-to-control grid feedback path of the oscillator tube, a neutralizing'or balancing condenser which may be disposed in another bridge armand connected in thecontrol grid-to plate circ'uitof the oscillator tube for neutralizing or balancing out the stray shunt capacitance associated with the piezoelectric crystal unit, and

condensers," inductors, or' resistors respectively.

disposed in the tworemaining bridge arms and having a tap connection therebetween connected with the groundedcathode electrode of the oscillator tube, a tun'able circuit tuned with respect to the harmonic'mode frequency of the crystal body, and"'suitable discriminator means associ ated'with the oscillator feedback path for effectiyely attenuating the undesired fundamental mode frequency of the crystalbodyor bodies while simultaneously transmitting the desired harmonic" mode frequency thereof, and if desired a suitable tuned output circuit for taking on from the crystal oscillator circuit any desired electrical trodes of the associated oscillator tube, may be.

utilized to prevent the circuit from oscillating at parasitio'frequencies. and hence to permit oscillations at" the desired harmonic mode series- 4 resonant frequency of the piezoelectric crystal body.

The impedance of the tank circuit utilized in the plate circuit of the oscillator tube may be kept high, and the impedance in the grid circuit thereof may be kept low, resulting in a maximum strength of oscillation and a decreased sheet of the oscillator tube upon the circuit oscillation frequency. Moreover, in circuits as here utilized, since the capacity of the crystal unit as measured from ground to-the control grid side of the oscillator tubedoes not act to prevent the circuit from oscillating because of degenerative feedback, any

, suitable "harmonic crystal may be utilized that has a reasonably'low shunt capacity across the crystal itself.

Anotherpoint of advantage is that, if the tube used is of the pentode type, an increase in the gain and an increase in the plate resistance for the oscillator tube; and also a decrease in the control grid-to-plate capacity thereof and-"hence 'a decrease in effect upon the oscillator circuitire quency results.

In order to'prevent oscillation-of the circuit asa grid-plate oscillator at the undesired fundamental "mode"frequency of "the piezoelectric crystal element, means may be provided to-pre-" sent a high series impedanceat the undesired fundamental mode frequency of-tlie piezoelectric crystal element thus in effect decoupling the o's cillator tube from the circuit at' that' frequency and thereby preventing circuit oscillations at that fundamental mode frequency, and 'at the 'same" time, this arrangement is'adapt'ed' topresent a much lower impedance at the'desired harmonic mode operating or working frequency o'fthe piezoelectric crystal element thus in effect conpling the oscillator tube to the circuit at that harmonic mode frequency and thereby permitting circuit oscillations at that desired harmonic mode frequency. Sucha discriminator 'ornlter' type arrangement may be conveniently usedwhen the lowest desired harmonic mcdeor'ierating'fre quency is relatively quite close to the highest un-f desired fundamental mode frequency or the piezoelectric crystal elementpan'd hence'may be used in applications Where'the use of other simpleexpedients would-be unsuitable to provide an ade -quate margin of frequency discrimination between the desired harmonic and the undesired fundamental mode frequenciesn Conversely; means may be provided to present a'low shunt impedance at the undesired fundamental-'mode ;frequency of the piezoelectric crystal element."

thus in effect decoupling the oscillator tube from the circuit atthat frequencyand thereby pre-l venting circuit oscillations at that fundamental" mode frequency, and at thesame time, this'arrangement is adapted t presenta much'higher shunt impedance at the desired higher harmonicmode operating or working frequency of the piezoelectric crystal element; thus in effect couplingthe oscillator tube ;-to the circuit at that harmonic mode frequency and thereby permitting circuit oscillations at that desired harmonic mode frequency. Also, these circuits may be made to operate over a range of desiredharmonio mode frequencies'due touse'of the discriminator methods referred to for preventing the undesired fundamental mode frequency oscillations. v

In order to'obtain a wide tuning rangefas'fo'r example to suit the range or harmonic mode frequencies that may be provided'by individually: utilizing in the circuit a plurality of, separate harmonic mode crystal bodies of different frepossible.

estimator circuit;- in ord r to provide multi frequency crystal-controlled channels for the? oscillator. The switching arr'angement may'be'of; the linown turrettype,- or may be a retr eteiy update-d arrangement utilizing suitablerelayst. The switches and crystals may be placed reia ticeiy close ts the oscillatbr tube in order to keep an shuntcapacities m the circuit as low m an" alternative arrangement the" for the several harmonic mode crystal switches bodies maycomprise germanium rectifier" type In order to obtain a still higher output free quency than may beconveniently obtained'directl'y' from the crystal oscillator" circuit prorlmt ihg'the frequency corresponding to'the harmonic.

nio'de frequency of the piezoelectric body, use may be i'na'de or" the non-linear properties oi'th'e oscillator tube" to further 'generate 'electricalliar monies of the" mechanical harmonic mode-ire quality; Such electrical harmonics may be selected' by meansof a known double-tunedtrans to inter or other harmonic selector arrangement adapted td be tuned to any odd or even order electrical harmoni'csuch as a econd or" third: order overtohe' of the" mechanical riarmenic metre frequency output that has been provided by the: harmohieerys'taloscillator circu t itself. In suclr anarrangement; a circuit in the outputof the oscillator tube is tuned to a'frequeiicy'n'f where n is avalue" 2', 3, 4; etc and where f'is-the operat ing' mechanical harmonic mode series-resonantfrequency of the piezoelectric crystal may: The desired electrical hermeru of the crystar har moiiic meaerre ueney may beselected cyan-cuteput cir uit which maybe cenneeteuin tactics;

arrangement between the plate electrode or the" eseniater' tube and the harmonic mcde'rrequefiey tank circuit.

For a clearer understanding of thenatm e of this invention and the additional advantages,- features and objects there'o'f, reference is inane to thdfollo'wing description taken in connection" with the accompanying drawing; n which use: reference characters re resent like: or similar parts and in which:

Fig. 1 is a circuit diagram illustratingabridge" type" harmonic crystal oscillator utilizing" capacitance tapped bridge configuration assc ciate'dfwi'th' a grounded cathode eieetronic'cseir ia'tor'tube', and an inductance type discrimma means for suppressing oscillations at the uridesired fundamental mode freduen'cyo'f the" harmeme crystal body; H

1A is a circuit diagram illustrating a series of different frequency harmonic mode erye'te ses applied to the oscillator circuit of Fig 1 at the left side of the broken line l/l/;

1B is a harmonic crystal oscillator circuit diagram, similar to those shown-in Figs. land 1A, but illustrating aninductancetapped bridge configuration; instead-of the capacitance tapped bridge configuration as shown in Figs; 1 and 1A;

Fig. 1C is a circuit diagram illustrating one means of obtaining electrical harmonics or 1 the age-mm rlia'r nioiiic mode'frequency from the circuit-Poi Fig l tattthe riglit -of thelline a-z;

Eigl 2 is-a circuitdiagram illustrating a modification of the harmonic crystaloscillator circuits shcwni nzFigsi Land 1A, as provided with rectifier type crystal switching means;

Fi'g'dsisa' circuit diagram illustrating a modifi cation of harmonic crystal oscillator circuit pro-: vided with frequency adjusting means;

Figiis-a' circuit diagram illustrating a 'modiil cation of theharmonic crystal. oscillator circuit shown in. Fig. 3;

Figs; 4A and-4B. are diagrams illustrating: a1- terhativdforms' that may be utilized forthe impedance Zi and Fig.. 5".is"acircuit diagram illustrating. a modification-orthe' harmonic crystal oscillator circuit shownin-Fig. 4", and which may be utilized-when crystal bodies having comparatively low harmonic mode frequencies are employed.

Referring to the drawing, Fig. 1 is a circuit diagram-illustrating a harmonic crystal oscillator circuit comprising generally an amplifier or sourc'eof high gain VI-, a bridge network circuit 5' Bpr'ovldedwith a tuned tank circuit T comprising capacitors'cland C5 and a variable inductor LP, and harmonic mode piezoelectric crystal body X1 or- X2, etcl, a resonant choke coil L2 adapted-"for preventing thecircuit from oscillat ing at the undesired fundamental mode frequency of the-harmonic mode crystal body XI or X2, etc., andah output circuit at terminals 12 and [3;

As shown in Fig. l, the" amplifier source of gain Vt may be provided in. the form of a single pentdde type vacuum tube electronic gain device VI havingla' grounded cathode electrode I con nected to ground 2 and heated by a suitable eatne'ue heater 3 which maybe energized by a; suitable battery, or other power supply source '4; an input or'control grid electrode 5 which may be connected through an inductance winding L2 and a suitable' grid leak resistor Re and by-pass condenser 69" with the grounded cathode elec trade I-,-- a screen grid electrode 6 which may be coilhectedthrough a suitable by-pass condenser GID- te thegrounde'd cathode electrode I, a suppressorgtid electrode 1 which may be connected with the-grounded cathode electrode I, and an anode or plate electrode 8 which, together with the cathode and control grid electrodes i and 5, mayconstitu'tethe oscillation generating electiod u orfthe vacuum tube VI. As shown in Fig';- l, the? anode" electrode 8' of the oscillator tube Vt: may: be" energized with a suitable positive potential; through the coil LI, the radio frequeneycnokeecu LF and a filter resistor Rm, by"- meansor a battery or other suitable power supply source I'D having its negative terminal connected to ground 2. Similarly, the screen elec' f trode 'fi of theoscillator tube Vi' may be energized with asmtabie positive potential through filter re'slstorsRtand Rm connected with the positive terminal (+0 of the power supply source It.

Suitable direct current blockingor by-pass capacitors- C9; C10; C11 and Ge may be provided as' sli'own in Fig. 1.

illustrated in Fig. 1 the frequency-cont'rolliiigi bridge networksystem B which is connected in circuit relation with the grounded cathode elec trade I; the control grid electrode 5' and the piate cr ancde electrode 8' or the vacuum tube V1; may be" provided with tank circuit capacitance dividiu capacitors C4 and C5 disposed crtwo 'adjacent arms respectively of the'bridg'e B and having a tap connection at 5 therebetween nausea connected'with' the grounded cathode electrode" I' of the vacuum tube VI. The 'series resonant harmonic mode crystal bodyXI or X2 or'X3, etc, disposed in a third arm of the bridge B, may be connected in the feedback path circuit extending between the control grid and plate electrodes and 8 respectively of the vacuum tubeVl, as shown in Fig. 1. In the fourth arm of the bridge B, a neutralizing or balancing capacitor C2 may be provided and connected in circuit between the control grid and plate electrodes 5 and 8 respectively of the vacuum tube VI, in order to neutralize or balance out the stray shunt capacity CI associated with the crystal body XI, X2 or X3,-

etc.

The tuned tank circuit T, as illustrated in Fig. 1, may comprise the series-connected capacitors C4 and C5 connected in parallel circuit relation with the variable inductance tuning winding LI, and may be tuned'with respect to a desired mechanical harmonic mode seriesresonant frequency of one or any of the piezoelectric crystal bodies Xl, orXZ, or X3, etc. Output circuit oscillations corresponding to the desired crystal harmonic mode frequency may be taken from the circuit at output terminals I2 and I3, the terminal I2 being connected with the grounded cathode electrode I of the vacuum tube VI, and the terminal i3 being connected through a suitable coupling condenser CI3 to the plate electrode 8 of the electronic gain device Vi.

The series-resonant harmonic crystal bodies XI, X2 and X3, etc., may comprise any suitable piezoelectric harmonic mode crystal bodies each adapted to operate and work at a desired mechanical harmonic mode overtone frequency of its fundamental mode frequency. For this purpose, a thickness shear mode AT-cut or BT-cut quartz crystal element, operating and working at an odd order mechanical harmonic of its fundamental thickness mode of motion, may be conveniently utilized, and the desired series resonant harmonic mode frequency thereof may be the third, fifth, seventh, or other odd order harmonic mode frequency thereof, corresponding to the frequency desired for the crystal oscillator circuit oscillations. Examples of such harmonic AT-cut or BT-cut quartz crystals, with mountings and holders therefor adapted to form a harmonic crystal unit, are disclosed in United States Patents No. 2,218,200 issued October 15, 1940, to Lack, Willard and Fair, and No.2,453,435 issued November 9, 1948, to H. Havstad; and in applications Serial No. 623,150 filed October 18, 1945, by L. J. La Brie, now Patent No. 2,486,482 dated November 1, 1949, and Serial'No; 26,013 filed May 10, 1948, by F. Caroselli, now Patent No. 2,509,478 dated May 30, 1950. "While the present invention is described particularlyin connection with AT-cut or BT-cutharmo'nic mode crystals which employ thickness mode vibrations of the shear type, it will be understood that the harmonic crystal bodies XI, X2, and X3, etc., may be any suitable type of harmonic crystal body where the capacity CI across the crystal itself is preferably of reasonable W value. Since, in this circuit arrangement as shown in the drawing, the capacity of the crystal unit XI, as measured from the control grid side 5 of the circuit to ground 2, does not tend to prevent the. circuit from oscillating because of degenerative feedback, the crystal body XI or X2, etc., may "be any suitable harmonic crystal body that has a reasonably low capacity CI across the crystal body itself. p

As illustrated in Fig. 1, the harmonic crystal body XI, X2 or X3, etc. operates at its harmonic; series-resonance frequency; and for operation at its series-resonance frequency the harmonic crystal body XI, X2 or X3, etc., is connected in circuit in the feedback path extending between the control grid electrode 5 and the plate electrode 8 of the vacuum tube VI, the circuit being traced as follows: from the control grid electrode 5 of the vacuum tube VI through the crystal body XI and the blocking condenser Cc to one end I5 of the tuned tank circuit T, then through the tank circuit T to the other end terminal I4 thereof, and then back to the plate electrode 8 of the vacuum tube VI.

As illustrated in the drawing, a single oscillator tube VI may be utilized, with a resultant reduction in the magnitude of current drain upon the associated power supply sources 4 and III, a feature which may be of interest in portable or mobile or other isolated installations, where a low'current drain may be desired or required.

Also as illustrated in the drawing, the use of the vacuum tube VI in the form of a pentode VI. isshown. The use of the pentode VI results not only in an increase in the gain and in the plate resistance for thevacuum tube VI, but also in a decrease in the value of the grid plate capacity. for the tube VI, which is effectively part of the neutralizing capacity C2, so that smaller effective values of neutralizing capacity C2 may be realized. Also, the use of the capacity tap at I5 between capacitors C4 and C5 on the harmonic crystal oscillator tank circuit T reduces the possibility of parasitic oscillations, since the vacuum tube VI and the stray capacity associated therewith is thereby paralleled with the capacitors C4 and C5 and not with parts of the tank circuit inductance LI. Accordingly, with the use of such a single tube VI having a grounded cathode electrode I connected to a tap I5 between the capacitance dividing capacitors C4 and C5, there is provided not only a resultant saving in the number .of tubes used, but also the magnitude and number of undesired response frequencies may be reduced,v and the possibility of obtaining oscillator operation on other than the correct frequency is reduced. The neutralizing or balancing capacitor C2, which as shown in the drawing, is connected in circuit between the control grid and plate electrodes 5 and 8 respectively of the vacuum tube VI, may be made up in its capacitance value of the grid-to-plate interelectrode capacity of the vacuum tube VI, plus an external capacity such that the bridge B is balanced against'the stray capacity CI that may be associated with the crystal unit XI, or X2, etc. The capacitance value for the neutralizing capacitor 02 may usually be quite small, even when the capacitance dividing capac itors C4 and C5 are chosen in the optimum ratio with respect to each other. Thus, the impedance of the tank circuit LI, C4, C5 in the plate circuit 8 of the vacuum tube VI, may be kept, relatively high and that in the grid circuit 5 thereof may be kept relatively low, thereby resulting in a maximum strength of oscillation and a decreased effect of the tube VI upon the desired oscillator fre quency value.

The tank circuit voltage-dividing capacitors Gland C5 may be chosen in the optimum ratio for oscillation by methods well known in the art, and the capacitances Cc, CI and C2 may be related thereto. in the following manner: The capacitance Cc serves to block the direct-current plate supply.

assesses voltage-from the crystal I and isnormally large in capacitance compared to the capacities Cl and C The capacity Cl is thesum capacitance of theshunt'capacity -of the crystal X I, its :holder,

and socket and stray wiring-andcircuit capacity.

The capacitance of C2 is then-chosen in value to balance the bridge composed of capacitances .C4,*-C5,"C| and C2, so :tha't no transmission :from cthe tplate circuit 8 to the grid circuit occurs because of presence of capacity Cl. Thus-if capacitances C4 and- C5-were in -the ratio of=2 tothe ratio of capacitances Cl to-C-Z should be 21,0 1. "Transmission 'f-ronrthe platecircuitt 'to -the gridecircuit 5 then-can only occur-When-the crystal: impedance, across which thecapacity Cl -appears, is low compared to that of CLwhich condition obtains a only when the -frequency is at or very close to the series-resonantfrequency of the-crystal XI and the circuit then oscillates at this frequency.

in accordance with -a feature of'this invention, 'the discriminator coil- L2,-which-in Fig.4 isadded incircuitbetween the-gridelectrode 5 oflthe-oscillator tube V! and the gridend of the grid leak resistor '.RG, may be utilized to prevent-circuit 'lowestqdesired harmonicmode irequeneyof the crystal bodies X1, X2, X3, -etc., and therebyis made-of asuitableimpedance value to attenuate orineffect -block the undesired lower frequency fundamental mode oscillation and, at the same time, transmit the desired-higher= frequency harmonic mode oscillations. Accordingly, the- -discriminator coil L2 may -be designedto; present a relatively low impedance at the undesired fun damental mode frequency of the crystal body or bodies XI X2-and X3, and thus-efiectively-d'ecouple the grid of the oscillator-tube VI from the tuning tank circuit 'LI ,-C4, C5=at-such undesired :fundamental ---mode frequency or frequencies of =the crystal bod-ies XI ;-X2,X3. But, atthe desired operating or working -f-requency, --whi'ch is -;the harmonic mode series-resonantfrequency of 1 the harmonic crystai-body X l X2- or K3, the discriminator-choke-coilm isdesigned to present a much higher impedanceand thus effectively couplethe oscillator tube V! -=to the tuning tank circuit'-Ll, C4,:C5 at such desired-harmonic mode frequency orfrequenciesofthe crystal bodies XI ,"X2 or :Accorcl-ingly, 'due'tothepresent method of using the --discrim-inator; inductor L2 for preventing fundamental mode oscillations while -at--the same time permitting harmonic'mo'de oscillations, *the circuit shown in-the drawing,- -utili'zing aplurality -of= harmonic crystal -bodies Xl X2, X3, etc., =for multifrequencychannel operation, may-be made to 'operate .over a range --of working harmonicmode frequencies. The-tank circuit T, comprising the parallel-connected inductance winding LI .and capacitors C land'C5,--may be-tunedto suit the harmonic. mode frequencies within-the range of P such: frequencies or optionally the tank circuitlT may be tuned-byad-justment of the variable inductance ofthe winding Ll Oscillations at theharmonicmode frequency may -betaken-off :"from theoutput circuit bymagnetic or-capaciti-ve couplingmethods well'known to the art.

- 10 -It-.will benot'ed-that the oscillatorapparatus, -asshown in Figs. 1 and LA, is adapted to. operate at comparatively highharmonic mode crystal frequencies; and sincethe oscillatortube-W oper- :ates as a non-linear device, harmonics of the harmonicmode frequencyare' generated and may be selected by suitable tuned circuits FL'I associated withtheoscillator tube VL-as shown -in Fig. 10, wherein the output-frequency at the output terminals 4'2 and 13 maybe 2, =3, --4, etc, times that of 1 the working harmonic mode frequencyof the crystal: body XI XZ, orzX3, etc.

lhe frequency stability of these circuits iscomparatively good inrespect:to frequencyshift'with changes in the voltage of-thebattery or power supply-1!}, with changesrof tubes Vi, and :with changes in the tuningof 1 the tank circuit L I ,5 Cl

and C5. 1

Fig.--1A,-as alreadyindicated, isa circuit diagram utilizing the same circuit as shown in Figrl, "but'illustratingmore particularly, at the left-side of the broken line y--y, the plurality of separate and difierent frequency harmonic modecrystals XI, X2, X3, X4, etc., which may be selectively connected,- oneat--atime, into the associated arin of the bridge B, by means of suitablesele ctive relay operated switches KI, K2, K3; K4, etc., respectively, whichmaybeselectivelyoperatedbya remote control-selector switchrsl asillustrated-in "Fig. 3 for eXampla-theneby to provide multifrequency crystal-controlledoscillationsin the same oscillator circuit, at anyone of-a series of desired crystal harmonic) mode af-requencies spaced at intervals extending overia selected band-or range ofifrequencies'.

As an: illustrative example in aparticular case for a harmonic crystal osc'illator' -circuit, constructed in accordanoewith Figs. l 1 and l A, -and *IC, and: utilizing sixindividual AT-cut har moriic mode crystals XI, X2, X3, etc., operating 7 separately at six different working -'thickness-mode mechanical (fifth) l harmonic frequencies spaced at intervals overv the range-of {frequencies extending about from 43 .to 48- megac'ycles per second, the vacuum tube V l -may -be a-pentode known as the.6AJ5 or othersuitablesourceof gain, the powersupplysource Itmay be-a batteryof about '60 volts direct-current potentialor other suitable source of powe1- supply, and the-component-circuit resistors, condensers and inductance windings thereof--may have values approximately as -follows --inductance choke coil LF about microhenries, tank circuit inductance tuning winding Ll-trom-about .48- -to .60 microhenry-as provided by about seven and one-half turns of-NoL 20 wire at eighteen-turns per inch on -a "0 37'5- 'in'ch diameter form tuned'with a -inch'by- %'-ineh long iron eslug, condensers :C9, Cl0, CI'I each about 1500 v-micromicrofarads, -capacitance :10! across the, crystals :Xl or -X2,r ,etc., about 4 micromicrofarads,neutralizing CQHGQHSGIXICL about 1.0 --micr9microf ar ad, ;,-,condenser Cc ;a b,out 010.0 mi rom c fa ads tan c r u --.cond nser a u 1-0- mi e ie qfa da ta zc re {cork denser C ab u rz imi em qs iacae sers. Q6 and Cl.;in,,this, instance about i microfarad when'thssecircuits associated tube capacities, ;Lgrid leak .esi'stdr R JG abou t 79 9 0 n ,fiLi I e i or R ri Bmeach about 1000,,ohms. The c us ns, c um, tunab t t I etim th requsncro tbeecry tal frequency tank "circuit Li, LCIZJIQ5," mayeach comprise about sixturns of, ,l lo. 1 8;wire ,atjour- -teen turns per inch on za -($35 0 inch diarneter'iorm tuned -with--a-'%*inch by %'-'inch long iron-slug,

:condenser C I 4.

,an'dradio frequency choke coil L2 may comprise, this particular example, about sixteen turns of No. '25 wire close wound on a 0.198-inch by inch long iron core with 'pigtail leads.

Fig. 1B is a'diagram of a harmonic crystal oscillator circuit wherein the configuration of the bridge or network B has been modified from the form utilizing the tapped bridge arm capacitors C4 and C5 as shown at I5 in Figs. land 1A, .to the form utilizing a tapped coil LI as shown at -I5 in Fig. 1B, the point I5 being by-passed to ground 2 by a condenser CM and the position of the grounded tap at I5 on the bridge B, and other connections to thebridge B, being otherwise the same as illustrated'in Figs. 1 and 1A. Accord- ;ingly, as shown in' Fig-1B, the two bridge arm capacitors C4 and 050i Figs. 1 and 1A have been removed and the tapped coil LI substituted therefor, and a capacitor C3 has been inserted between ,points I4 and I6 in the place of the coil LI in Figs. '1 and 1A, the tank circuit T of Fig. 13

thereby comprisingthe capacitor C3 shunted by the variable inductance coil LI tapped at I5 and connected with the grounded cathode electrode ,I of the oscillator tube VI through the by-pass ;As compared with the tapped inductance form of bridge circuit B illustrated in Fig. 1B, the

tapped capacitance form of bridge B illustrated in Figs. 1 and 1A results in somewhat more freedom from parasitic oscillations and also in lower losses in the tuned tank circuit T, because of somewhat less circulating current in the ground path thereof at I5. The capacitance tapped bridge B shown in Figs. 1 and 1A has however a somewhat greater tendency to oscillate at the undesired fundamental mode frequency of the associated crystal body XI, X2 or X3, etc., and .in order to block such oscillations at that undesired fundamental mode crystal frequency referred to, the gain at that frequency, around the feedback loop circuit, maybe reduced at that fundamental mode frequency to less than unity by suitable means such as by tapping the plate electrode 8 downon the tuned circuit T by inserting a ,small capacitanceinot shown) between the plate electrode -8 and the top of the-tuned circuit at I 4, the shunt feed choke Lp-then'being disconnected from IB and instead connected to the electrode a of the tube V! or by shunting the grid 5 to ground-2 by means of the discriminator inductance winding L2, the inductance value of the winding L2 being chosen to present a low value of reactance at the undesired fundamental mode crystal frequency while still presenting a high impedance to the desired harmonic mode crystal frequency or frequencies. For this purpose the value selected for the inductance winding L2 may be such as to just parallel-resonate the total circuit grid-to-ground capacity Cg at a frequency slightly below the lowest harmonic mode crystal frequency to be used, and such a selection insures that the sign of the reactance 'from the grid 5 to the ground 2 over the desired harmonic mode crystal frequency band may remain the same and thus prevent the possibility of other or spurious oscillations.

While the discriminator inductor L2 may be utilized in combination with the capacitance tapped bridge B shown in Figs. 1 and 1A, or with the inductance tapped bridge B shown in Fig. 13, its use in connection with the latter form shown in Fig. 13 may be dispensed with in cases where the plate 8 to cathode I impedance at the undesired fundamental mode crystal frequency; is suiliciently low. i

Fig. 2 is a diagram illustrating a harmonic crystal oscillator circuit, similar to that shown in Figs. 1 and 1A provided with one form of remote switching means, but provided in Fig. 2 with a modification in the form of the remote switching means wherein the crystal selector switching means may comprise rectifiers YI, Y2, Y3, Y4, etc. adapted to permit the remote selection of any of the several mechanical harmonic mode series-resonant frequencies contained in a desired frequency band as provided by the several harmonic mode crystal bodies XI, X2, X3, X4, etc. As an illustrative example, the desired frequency band may be a band such as an 0.5 per cent frequency band with a spacing'between the frequen- 'cies thereof of about 0.07 per cent.

As illustrated in'Fig. 2, the circuit may employ a bridge or network B having selectively disposed in one arm thereof a plurality of harmonic mode crystal bodies XI, X2, X3, X4, etc., and a plurality of associated rectifiers YI, Y2, Y3, Y4, etc. respectively, provided with suitable switching equipment for selectively and operatively disposing any of the harmonic mode crystal bodies XI, X2, X3 or X4, one at a time, into the bridge B. The bridge B in Fig. 2 may, like the bridge B in Figs. 1 and 1A, be connected with the grounded cathode oscillator tube VI, and the circuit "of Fig. 2 may, similar to that in Figs. 1 and 1A, provide harmonic mode crystal-controlled oscillations with suppression of the undesired fundamental mode crystal frequencies by means of the discriminator inductance Winding L2, or by other suitable means. Like reference characters in Fig. 2 represent like or similar parts in Figs. 1 and 1A, the additional parts in Fig. 2 comprising a seriesconnected inductance winding L and resistor R associated with the bridge system B, a plurality of rectifiers YI, Y2, Y3, Y4, etc., and respective radio frequency choke coils LFl, Lr'z, Les, Lm', etc., associated with the crystal selector switching device S. The rectifiers YI, Y2, etc., may be, for example, any suitable type of germanium crystal diodes. The winding L may also be a radio frequency choke coil. These choke coils are selected to present negligible shunting effect upon the portion of the tuned circuit T across which they are connected. The resistor B. may be any suitable resistor having a resistance value selected to limit the direct current through the crystal diodes YI, Y2, etc. to their rated value,*and the switch S may be a suitable crystal selector switch which selectively applies positive voltage from a suitable power supply source 20 through a suitable choke coil LFl, Lr'z, Lrs or LF4, etc., re-

spectively to the particular crystal diode YI, Y2,

etc., associated with the then working respective harmonic crystal body XI or X2, etc., of the desired harmonic mode frequency, and which at the same time grounds all other crystal diodes YI, Y2, etc., associated-with the remaining harmonic mode crystal bodies not then operating.

When the crystal selector switch S supplies a positive voltage to a particular crystal diode YI or Y2, etc., that diode then becomes electrical- 1y conductive and will have an internal forward resistance which may be of the order of approximately 75 ohms, or other suitable resistance value, which is low enough to permit the circuit to oscillate at the harmonic mode frequency of the particular harmonic crystal body XI or X2, etci, associated with the then working crystal diode YI or Y2, etc. At the same time, since the point arses-gees '-Z' inthe circuit is positive with respeotto ground 12 and the selector switch S applies agroundconnection to the remaining crystal diodes, the latter are not then electrically conductive since they then have a high reverse impedance with the result that the circuit will not then oscillate at the frequency of an of the other remaining harmonic crystal bodies. Thus, as an example as illustrated in Fig. 2, the selector switch-S when disposed in the position particularly shown therein applies a positive direct-current potential from the power supply source 20 to the diode Y2 only, thereby permitting the associated piezoelectric harmonic crystal body X2 to oscillate; at the same time, the selector switch S connects to ground 2 all of the remaining diodes Yi, Y3, and Y4, etc., thereby preventing theassociated piezoelectric harmonic crystal bodies XI, X3, and X4, etc., from oscillating.

' It will be understood that the capacitance value of the neutralizing or balancing capacitor C2 of Fig. 2 may be increased somewhat over that shown for use in the circuits of Figs. 1 andlA, in order toneutralize the. increased capacity as provided-in Fig. 2 by the additional diodes YI,

Y2, etc. that are utilized in Fig. 2.

Fig.3 is a diagram illustrating a bridge or network type harmonic crystal oscillator circuit, provided with a small amount of circuit frequency shift or adjustment individually for each of the plurality harmonic mode crystal bodies XI, X2, X3,. etc., which are adapted to operate separately :at their respective different harmonic mode frequencies spaced at intervals extending over a selected range or band of frequencies.

.The small amount of adjustable shift inthe crystal oscillator frequency may be obtained, as

with the respective coupling capacities C01, C0

Cw, etc. The respective inductances LIB, LII, LI2, etc., are connected inseries relation with the respective harmonic mode crystals XI,-X2, X3, etc., and are disposed in the feedback path circuit extending between the control grid electrode 5 and the plate electrode 8 of the oscillator tube VI. cuit (LIO, C01, Cg; LI I, C02, 0;; LI2, C03 Cg, etc.) arrangement results in a substantially QO-degree phase shift in the oscillator feedback loop circuit, the additional QO-degree phase shift required in the feedback path for oscillation at the bridge terminals I4 and I6, may be obtained by means of an inductance device L9 having a high leakage inductance and adapted for coupling the bridge B to the plate circuit 8 of the oscillator tube VI. The inductive coupling as provided by the in- Since the use of this tuned cirductance windings L8 and L9 and the low value of resistance as provided by the bridge arm resistors RI and R2 of the bridge B may be made of values to make the gain of the pscillator feedback path too low for oscillation at the undesired fundamental mode frequency of the harmonic crystal body XI or X2, etc., and hence the .use of the inductance winding L1 is not necessarily required here, in most cases, to stop oscillations at the undesired fundamental mode crystal frequency but may however be used for that purpose if desired, as well as for the purpose of raising the effective capacitive reactance at the input grid circuit 5 of the oscillator tube VI so that an increased range of frequency adjustment is providedby the associated series inductance tun- 1118.

As illustrated in Fig. 3, the plurality of series resonant harmonic mode crystal bodies XI, X2, X3,"etc. associated with their respective bridges BI, B2, B3, etc., may be individually connected in the feedback circuit extending between-the control grid and plate electrodes .5 and 8 respectively of the pentode type oscillator tube VI by means of the respective crystal selecting relay operated switches :KI, K2, K3, etc., controlled by a suitable crystal relay selector switch SI.

For the purpose. of independently adjusting'the circuit frequency associated with eachof the :harmonic mode crystal bodies XI, X2, X3,. etc., the frequency-adjusting variable inductance .devices L10, LII, Ll2, etc.,-may each be connected in series circuit relation with the respective harmonic'mode crystal bodies XI, X2,.X3,:etc.. .and may'each be varied in inductance value, as for example some :30 per cent, :by means of a known powdered iron slug inductance control device. Connected inseries circuit relation with the respective frequency-adjusting inductances LII], LII, LI2, etc., there may be provided condensers Col, C172, Cc3, etc., respectively, which may each have a capacitance value of the order, of 5 .micromicrofarads or other suitable value adaptedto raise the inductance-capacitance ratio of .the frequency-adjusting circuit LIILCci, etc., to

a value such that the inductance adjustment of the respective inductors LIIl, LI I, LI2, etc., may shift the circuit frequency-agreater amount.

As illustrated in Fig. 3, the respective bridges BI, B2, B3, etc., may each comprise four arms, the two lower-arms of which may comprise the common resistors RI and R2 having a tap :connection I5 therebetween connected with the grounded cathode electrode I of the oscillator tube VI. The resistors RI and R2 may be made of suitable resistance values to develop thereacross at terminals I4 and t6, thegrid driving and neutralizing voltages for the circuit. It will be noted that in the circuit of Fig. 3, the bridge arm resistors RI and R2 areutilized, in place of the bridge armcapacitors C4 and C5 shown in Fig'sgLlA and12.

. As illustrated in Fig.3, neutralizing or balanczing capacitors Cm, Cm, Cm, etc., similar to the neutralizing capacitor 02 shown in Figs. 1 and 2, may be associated with the corresponding arms of the respective bridges BI, B2, B3, etc., and may be utilized to prevent the circuit from'oscillating at other than the desired series-resonant frequency of the respective circuits associated therewith, by neutralizing or balancing out the stray shunt capacity CI associated with the respective harmonic crystal units XI, X2, X3, etc.

The inductance coil L'lycorresponding to the discriminator inductance coil bearing the reference character L2 in Figs. 1, 1A, 1B and '2, may be similarly utilized to parallel-resonate the input circuit capacity of the tube VI and also "the capacity of the'associated relays KI, K2, K3, etc., at a frequency which is somewhat below the lowest harmonic mode frequency of the series of harmonic crystal'bodies XI, X2, X3, etc., thereby to reduce the elfective grid capacity at the harmonic mode frequencies of the crys'tal'bodies XI, X2, X3, etc., and to suppress oscillations'at --'the undesired fundamental mode frequencies thereof. Alternatively, the inductance winding LI may be utilized in the circuit shown in Fig. 3 merely to raise the impedance of the input'grid circuit 5 of the oscillator tube VI so that the inductance-capacitance ratio of the associated frequenoy adjusting circuits may be increased and oscillations at the undesired fundamental mode a greater frequency shift obtained by, adjustment ,of L10, Ll l, etc., while relying upon the inductive .coupling of windings L8 and L9 and the ;lo w resistance valuesof resistors Rl and R2 to stop frequencies of the crystal bodies XI, X 2;an d;X3, etc.. :1; As an illustrative example, the harmoniccrystalbodies XI, X2, X3, etc., mayinFig. 3 comprise six or other number of difierent frequency hare .monic mode series-resonant typecrystal bodies, and may be operatedwithout the usenof a con,- stant temperature-controlled heating oven with a considerable saving in current drain on the associated power supply source, a factor which may be of interest in. connection with. use, in yehicular .or other mobile .or isolated. radio equipment. Where thev harmonic. crystal .bodies- .';Xi, X2,,X3. etc., have a good frequency stability as of the order of $005 percent, they. may. be utilized :without the heating oven,. and the circuit. illustrated in Fig. 3 may provide a frequency achustment of the order of $0015 per. cent. thereby eliminating the usual crystal grindingtolerance.

Asan illustrative example in a particular .case, a harmonic crystal oscillator circuitconstructed in accordance with the circuit: shown in Fig; 3 and providing a frequency adjustment of -about -i.00l5 per cent utilizing six harmonicmodecrystal bodies Xl, X2, X3, etc., havingaifrequency stability of the order. of :005 per cent .andgiving six crystal-controlled mechanical harmonic mode frequencies within a 220-kilocycle per second frequency band in a range of harmonic mode crystal frequencies from. about 43.8 to.'47;2 megacycles per second, mayhave the. following circuit components: the harmonic crystal .bodie'sXl X2, "X3, etc., may be .six separate AT-.-.cut harmonic mode quartz crystal piezoelectric elements having mechanical harmonic mode frequencies ofcabout 43.80, 43.82, 43.86, 43.90, 43.94 and 44.02. megacycles per second, respectively; The six .corre sponding frequency adjusting inductan'ces .Llfl, -LI I, L12, etc., may each have an inductance value of about 6.5 microhenries and maybeivaried in inductancevalue approximately :30 per cent by means of a known powdered ,iron slug. flhegsix corresponding series condensers Ca, Caz, Cc'a'etc" may each have a capacitance value of about?) micromicrofarads, in order to raise the. induct- $30 9 5 W is i e a le. 1 5. that. 2? effective grid capacity may be reduced to-approximately 5 micromicrofarads at the crystal working frequencies. The by-pass condensers C9 andCHl may each comprise 500-micromicrofar ad known button-type by-pass condensers. The-grid resistor'R maycomprise a grid leak resistor R3 of about 270,000 ohms, and a resistorRQ; of about 10,000 ohms, providing a metering pointer, .13 for grid current measurement. The oscillator .tube V! may be a pentode known as a 403i; vacuum tube, or other suitable electronic gain device, e nergized by a voltage of about lzwvolts from the power supply source 10 supplied through .a plate and screen filter resistor RF of about 1000 ohms; The plate tank tuning capacitor-CA, may havea capacitance value of about 20 micromicrofarads, and the plate tank tuning inductance L8 may have an inductance value of about 0.50 .microhenry. The grid circuit coupling coil L9 may have an inductance value of about 0.80 'microhenry or other value suitable forproviding a substantially QO-degree phase shift in that part vof the feedback path coupling the tank circuitT to the input of the bridges Bl, B2, B3, etc.

The plate tank circuit T at CA, L8 may. be turned to resonance at the mean frequencyof the several harmonic crystal bodies which are six in number as utilized in the foregoing example. Since, in that example, the maximum spreadin crystal frequency is about 220 kilocyclesper sec;- ond', the plate tank circuit CA, L8 .will betuned -to resonance within about $.25 per cent'for any crystal frequency, and is therefore predominately resistive. The grid coupling coil L9 has a'leakage reactance of approximately. ten times the termiance-capacitance (L/C) ratio of each associated respective crystals and their sockets associated with the six corresponding harmonic crystal units XI, X2, X3, etc., thereby to prevent the circuit from oscillating at other than the desired series resonant crystal frequencies thereon The bridge arm resistors RI and R2 may be about lQ-oh carbon resistors. The six corresponding crystal selecting relay switches Kl, K2, K3, etc., maygbe any suitable devices. The coil L1 may havean inductance value of about 1.5 microhenries or other value designed to parallel-resonate thevacuum tube VI input capacity and the relay Ll, etc., capacity totaling about 16 micromicrofarads, at a frequency somewhat below the lowest crystal nating resistance of resistorRl plus resistor. R2, and is coupled to the plate tank tuning inductance L8, and when it is so terminated in the bridge resistors RI and R2, the Q of the plate tank circuit tuning inductance L8 is reduced to approximately 50 per cent'of its value when not so terminated. Accordingly, because the leakage reactance of .the 'grid circuit coupling coil L9 is made relatively high as compared to the resistance of resistor Rl plus resistor R2, the voltages developed across the series resistors RI plus R2 are essentially at '90 degrees phase relation with respect to thevoltage across the plate tank circuit tuning inductance L8. The relatively low values of resistances employed for the terminating bridge resistors R1 and R2 insure that this QO-degree phase shift may be obtained even though the capacity across the coupling coil L9 as provided by the crystal bodies X1, X2, X3, etc., and also by the neutraliz -ingcondensers Cm, 0N2, Cm, etc., may be high,

The frequency-adjusting variable inductance coils L10, L! 1, L12, etc., tune the various 'grid circuits respectively associated therewith to series "resonance and present a resistive load to there- "spective crystal bodies X1 X2, X3, etc., which also operate at series resonance. Since the voltage on the grid electrode 5 of the oscillator tube Vi is developed across the efi'ective grid capacity Cg, a further JO-degree phase shift occurs therein, and accordingly the oscillator feedback loop circuit then has a full -degree phase shift required for, oscillation.

When the frequency-adjusting inductance coil LIB, Li I, or L! 2, etc., is adjusted so that theassociated external grid circuit operates on either 'side 'of the series resonance condition, a varying value of capacity or inductance is then presented to the associated crystal body X1, X2, X3, etc., which harmonic mode frequency of about @318 mage} 5s 'then 'shifts its operating frequency slightly until s nses it supplies the inductance or capacity required for the correct oscillator phase shift and maximum grid voltage for the oscillator tube VI. The amount of such frequency shift that can be obtained in this manner is essentially limited by the crystal body XI, X2, X3, etc. In one direction of operation, namely, as the magnitude of the inductance of the variable inductance device LII), LII or LE2, etc., is increased in inductance value, the associated crystal body XI, X2 or X3, etc., operates at a frequency at which it is in effect a smaller and smaller capacity until it finally becomes comparable to the crystal shunt capacity and at that time the circuit becomes neutralized by the associated neutralizing condenser Cm, Cm, Cm and oscillation ceases. In the other direction of operation, namely, as the magnitude of the inductance of the variable inductance device LIO, LI I or LI2, etc., is decreased in inductance value, the associated crystal body XI, X2 or X3, etc., operates at a frequency at which it is, in effect a larger and larger inductance since the external associated circuit then presents in effect a smaller and smaller capacity. However, this capacity can never be less than the effective crystal capacity to ground 2 at the bridge junction I8 and accordingly when that condition is reached, any further decreases in the inductance of LII), LI I, LIZ, etc., produce essentially no further change in the oscillator frequency.

Fig. 4 is a circuit diagram illustrating a modification of the harmonic mode crystal oscillator circuit shown in Fig. 3, and providing among other points of difference, a simplified form for the bridge system B over that shown in Fig. 3, a simplified form of crystal switching arrangement SI for the harmonic mode crystals XI, X2, X3, etc., and feedback path phase shifting means comprising bridge coupling capacitors CI I and CI 2, in place of the inductance coupling L9 shown in Fig. 3, for phase shift purposes and other purposes such as preventing oscillations at the fundamental mode frequencies of the crystal bodies XI, X2 or X3, etc.

As illustrated in Fig. 4, the harmonic crystal oscillator circuit may comprise a bridge or network B connected in circuit relation with the plate or anode electrode 8, the input or control grid electrode 5 and the grounded cathode electrode I of the pentode type oscillator tube VI.

The several series resonant harmoni mode series-resonant type piezoelectric crystal bodies XI, X2, X3, etc., may each be selectively and separately disposed in one arm of the bridge B in the feedback circuit extending between the control grid and plate electrodes 5 and 8 respectively of the oscillator tube VI, by means of a single pole multiple contact position switch SI. The neutralizing capacitor CN may be disposed in another arm of the bridge B and may, as hereinbefore described, be utilized, when needed, in order to neutralize or balance out the shunt capacity across the crystal units XI, X2 or X3, etc., and thereby prevent the circuit from oscillating at frequencies other than the desired series-resonant harmonic mode frequency of the crystal body XI, X2 or X3, etc. The resistors RI and R2 may be tapped therebetween at I5 and connected with the grounded cathode electrode I of the vacuum tube VI, as hereinbefore described in connection with Fig. 3.

The resistors RI and R2 are resistors across which may be developed the grid-driving and neutralizing voltages for the bridge circuit B, and

the phase shifting, bridge coupling capacitors CI I and CI2 which in turn may be connected with the opposite ends of the plate circuit tuned tank circuit T comprising the series-connected plate circuit tuning capacitors CI3 and Old shunted by the variable plate circuit tuning inductance winding L8.

As illustrated in Fig. 4, the input grid circuit 5 for the vacuum tube VI may include therein, connected in series circuit relation, a variable grid circuit tuning inductance winding LE3, a grid circuit direct-current blocking and radio frequency current coupling capacitor Cc, and a frequency-adjusting impedance device ZI which may comprise either an inductance device as illustrated in Fig. 4A or a capacitance device as illustrated in Fig. 43, depending upon the particular mechanical layout of the system utilized, as hereinafter described more fully.

The circuit, as shown in Fig. 4, may also include an impedance limiting resistor R5, a pair of series-connected grid leak and grid current measuring resistors RS and R4 respectively, a common screen and plate circuit filter resistor Re, a radio frequency choke coil LP disposed in the circuit supplying positive potential to the plate electrode 8 of the vacuum tube Vi from the power supply source It, and a common screen and plate circuit by-pass capacitor CH]. The effective input-to-ground 2 capacity for the vacuum tube VI may be represented by the capacitance Cg, as shown in dotted lines in Fig. 4. This capacity Cg cooperates with the input circuit tuning inductor LI3 in order to provide a substantially EDD-degree phase shift in the oscillator feedback path, as described more fully hereinafter.

In operation, the tuned plate circuit L8, CI3, Cid for the oscillator tube VI may be tuned to resonance at the mean frequency of the several harmonic mode crystal bodies XI, X2 and X3, etc., that may be utilized to provide a plurality of harmonic mode crystal frequencies having a desired spread in crystal frequencies, as of about :25 per cent for example. Where the maximum spread in crystal frequency is of the order of that value, the plate circuit impedance of the oscillator tube VI will be predominately resistive in character. The bridge resistors RI and R2 may be chosen in value to be substantially lower than the reactance of any stray circuit capacity disposed across them, and the capacitors CH and CI2 may be chosen to have a substantially higher value of reactance than the resistances of RI and R2 respectively. The voltages applied at points I4 and I6 across the series-connected resistors RI and R2 will then be essentially 180 degrees out ofphase with respect to each other, and also essentially at a QO-degree phase shift relation with respect to the voltage across the tuned plate circuit T. Accordingly, the phase shift capacitors CH and CI2 provide a substantially -degree phase shift in that part of the oscillator feedback path disposed between the output of the oscillator tube VI and the input to the bridge B.

Such use of capacitors CH and CI2 in conjunction with the resistors RI and R2 also acts to suppress fundamental mode oscillation.

The bridge arm neutralizin capacitor CN may be chosen in capacitance value to balance the bridge circuit B against the shunt capacity of each of the crystal bodies XI, X2 and X3, etc.,

XI, X2 or X3, etc., as selected by the switch S1, since the bridge circuit B is unbalanced at this series-resonant frequency only. The voltage appearing across the impedance limiting resistor R is stepped up at the grid electrode 5 of the Vacuum tube Vi, by the resonant circuit comprising the inductor Li3 disposed in the grid circuit and the associated grid 5-to-ground 2 capacity Cg, and results in an additional feedback path 90 degrees phase shift of the voltage at the grid 5 of the tube Vi, thus giving the necessary 180 degrees total feedback path phase shift required for oscillation.

The value of the impedance limiting resistor R5 may be chosen to be substantially lower than the reactance of any stray circuit capacity across it, and thus may serve to limit the impedance presented to the crystal body XI, X2 or X3, etc., by the detuning of the LI3, Cg circuit that would otherwise produce a frequency change in the circuit, in the manner of the frequency changing circuit at LI3, L! l, Ll2 described in connection with Fig. 3. The tuned L53, Cg circuit in Fig. 4 may be normally tuned for resonance at the mean frequency of the several harmonic mode crystal bodies Xi, X2 and X3, etc., utilized in the circuit of Fig. 4, as indicated by the maximum oscillator grid current measured at H).

The impedance Z! of Fig. 4 may be either an inductive or capacitive reactance device as illustrated .in Figs. 4A and 4B respectively, and as shown in Fig. 4 may be connected in series circuit relation with the bridge B in order to adjust the frequency of oscillation in this circuit of Fig. 4 to correspond, for any particular crystal body Xi, X2 or X3, etc., to that of a standard frequency crystal test set (not shown) utilizing the same crystal body, for frequency correlation purposes. The impedance Zl accordingly depends on the particular mechanical layout of this circuit compared to the associated test set arrangement, and may be chosen so that, when the L33, Cg circuit of Fig. 4 is tuned for maximum oscillator grid current therein as measured at l9, the abovementioned frequency correlation is obtained. It will be noted that the impedance Z1 of Fig. 4 acts in the manner of the frequency adjusting seriesresonant grid circuit employed in the oscillator shown in Fig. 3.

It will be understood that the values of the component elements utilized in the circuit of Fig. 4, may be chosen to suit the characteristics of the particular crystal bodies Xi, X2 and X3, etc., utilized therein. For example, when utilizing harmonic mode crystal bodies Xi, X2 and X3, etc., operating at the relatively lower ranges in frequencies, as from about to 70 megacycles per second for example, the crystal series-resonant resistance may then be low compared to the value of the crystal shunt capacitive reactance so that, when operated between the relatively low impedance resistors RI and R5, energy passed through the crystal shunt capacity may then not be sufficient to sustain circuit oscillation while energy passed through the relatively lower crystal series-resonant resistance will sustain oscillation. The capacitors CN and CI2 and the resistor R2 of Fig. 4 may then be omitted from the circuit as illustrated in Fig. 5. However, the latter circuit components Cu, Cl2 and R2 would be used and included as illustrated in Fig. 4 at some higher frequency where the crystal shunt capacitive reactance is lower as compared to the crystal series-resonant resistance.

Accordingly, the crystal bodies XI, X2 and X3,

etc., may, in the circuits illustrated in Figs. 4 and v 5 be operated between low impedances as provided by resistors RI and R5 at the lower values of crystal frequencies with the possibility of eliminating the neutralizing components CN, Cl2, R2 as illustrated in Fig. 5; or with the possibility of using such neutralizing components (In, Cl'2 and R2 where needed, as at the higher values of crystal frequencies as illustrated in Fig. 4.

It will be noted that the circuits shown in Figs. 4 or 5 may employ a single pentode tube V! with a resultant saving in power supply current, that impedance means Z! may be provided in the grid circuit 5 for adjusting the circuit frequency, and

'- that the increased gain in the feedback path to the grid electrode 5 as provided by the step-up in gain from the LI3, Cg circuit, permits otherwise marginally active crystals XI, X2 and X3, etc. to oscillate at good strength. The strength of crystal oscillation may be a matter of importance where it is desired, as in many cases, to generate an electrical harmonic or overtone of the crystal harmonic mode oscillation frequency in the oscillator tube Vi itself, and take oif this higher frequency electrical harmonic frequency energy for utilization, as by means of an electrical harmonic tuned circuit disposed in series with the plate electrode 8 of the oscillator VI and tuned to the desired electrical harmonic frequency referred to, in the manner illustrated in Fig. 1C. In general, the greater the strength of crystal oscillation, the greater will be the magnitude of oscillator grid drive and electrical harmonic ouput produced in the oscillator tube VI.

Also, it will be noted that in the circuit illustrated in Figs. 4 or 5, .means for obtaining a substantially QO-degree phase shift in the feedback path plate circuit coupling to the bridge or network Bare provided, in combination with the tuned grid circuit of the grounded cathode tube VI and that the single tube (VI) circuit incorporates means for operating the crystal body X I X2 or X3, etc., between low values of impedances R! and R5. I

Although this invention has been described and illustrated in relation to specific arrangements, it is to be understood that it is capable of application in other organizations and is therefore not to be limited tothe particular embodiments disclosed.

What is claimed is:

1. Harmonic crystal oscillator apparatus comprising an electronic gain device having oscillation generator electrodes comprising anode, control grid and grounded cathode electrodes, a bridge connected in circuit relation with said last-mentioned electrodes, a mechanical harmonic mode series-resonant frequency type piezoelectric crystal body disposed in one arm of said bridge in the feedback path of said gain device extending between said anode and grid electrodes thereof, a circuit including another and capacitive arm in said bridge disposed adjacent said first-mentioned bridge arm, connected at its ends in circuit between said anode and grid electrodes of said gain device and constituting means for neutralizing or balancing out the shunt capacitance disposed across said harmonic mode crystal body, a tuned tank circuit reactance means connected in shunt relation across said two adjacent arms of said bridge, disposed in said feedback path and tuned substantially to said mechanical harmonic mode series-resonant frequency of said crystal body, and discriminator reactance means disposed in circuit relation with respect to said feedback path for transmitting oscillations through said gain device at said desired harmonic mode series-resonant frequency of said crystal body while simultaneously blocking or attenuating oscillations at the undesired fundamental mode frequency thereof.

2. Harmonic crystal oscillator apparatus com- .prising an electronic gain device having oscillation generator electrodes comprising anode, control grid and grounded cathode electrodes, a bridge connected in circuit relation with said lastmentioned electrodes, a mechanical harmonic mode series-resonant frequency type piezoelectric crystal body disposed in one arm of said bridge in the feedback path of said gain device extending between said anode and grid electrodes thereof, a circuit including a capacitive arm in said bridge disposed adjacent said first-mentioned bridge arm, connected at its ends in circuit between said anode and grid electrodes of said gain device and constituting means for neutralizing or balancing out the shunt capacitance disposed across said harmonic mode crystal body, a tuned tank circuit reactance means connected in shunt relation across said two adjacent arms of said bridge, disposed in said feedback path and tuned substantially to said mechanical harmonic mode series-resonant frequency of said crystal body, and discriminator reactance means disposed in circuit relation with respect to said feedback path for transmitting oscillations through said gain device at said desired harmonic mode series-resonant frequency of said crystal body while simultaneously blocking or attenuating oscillations at the undesired fundamental mode frequency thereof, said discriminator reactance means comprising an inductance device.

3. Harmonic crystal oscillator apparatus comprising an electronic gain device having oscillation generator electrodes comprising anode, control grid and grounded cathode electrodes, a bridge connected in circuit relation with said lastmentioned electrodes, a mechanical harmonic mode series-resonant frequency type piezoelectric crystal body disposed in one arm of said bridge in the feedback path of said gain device extending between said anode and grid electrodes thereof, a circuit including another or capacitive arm in said bridge disposed adjacent said first mentioned bridge arm, connected at its ends in circuit between said anode and grid electrodes of said gain device and constituting means for neutralizing or balancing out the shunt capacitance across said harmonic mode crystal body, a tuned tank circuit reactance means con-- nested in shunt relation across said two adjacent arms of said bridge, disposed in said feedback path and tuned substantially to said mechanical harmonic mode series-resonant frequency of said crystal body, and discriminator reactance means disposed in circuit relation with respect to said feedback path for transmitting oscillations through said gain device at said desired harmonic mode frequency of said crystal body while simultaneously blocking or attenuating oscillations at the undesired fundamental mode frequency thereof, said discriminator reactance means comprising an inductance device connected in shunt relation with respect to said grid and grounded cathode electrodes of said gain device and having an inductance value sufficient for parallel-resonating the circuit capacity disposed across said grid and grounded cathode electrodes at a frequency value of such a relation to said desired 22 1 harmonic mode frequency to thereby constitute said discriminator reactance means.

4. Harmonic crystal oscillator apparatus comprising an electronic gain device having oscillation generator electrodes comprising anode, control grid and grounded cathodeelectrodes, a bridge connected in circuit relation with said lastmentioned electrodes, a -mechanical harmonic mode series-resonant frequency type piezoelectric crystal body disposed in one arm of said bridge in the feedback path of said gain device extend- 7 ing between said anode and grid electrodes thereof, a circuit including a capacitive arm in said bridge disposed adjacent said first-mentioned bridge arm, connected at its ends in circuit between said anode and grid electrodes of said gain device and constituting means for neutralizing or balancing out the shunt capacitance disposed across said harmonic mode crystal body, a tuned tank circuit reactance means connected in shunt relation across said two adjacent arms of said bridge, disposed in said feedback path and tuned substantially to said mechanical harmonic mode series-resonant frequency of said crystal body, and discriminator reactance means disposed in circuit relation with respect to said feedback path for transmitting oscillations through said gain device. at said desired harmonic mode frequency of said crystal body while simultaneously blocking or attenuating oscillations at the undesired fundamental mode frequency thereof, said discriminator reactance means comprising an inductance device connected in shunt relation with respect to said grid and grounded cathode electrodes of said gain device and having an inductance value sufficient for parallel-resonating the circuit capacity disposed across said grid and grounded cathode electrodes at a frequency value of such a relation to said desired harmonic mode frequency to thereby constitute said discriminator reactance means, and a grid leak resistor paralleled by a by-pass condenser and connected in series circuit relation between said discrimina-i tor inductance device and said grounded cathode electrode of said gain device.

5, Harmonic crystal oscillator apparatus comprising an electronic gain device having oscillation generator electrodes comprising anode, control grid and grounded cathode electrodes, a bridge connected in circuit relation with said lastmentioned electrodes, a mechanical harmonic mode series-resonant frequency type piezoelectric crystal body disposed in one arm of said bridge in the feedback path of said gain device extending between said anode and grid electrodes thereof, a circuit including another and capacitive arm in said bridge disposed adjacent said first-mentioned bridge arm, connected at its ends in circuit between the anode circuit and grid electrode of said gain device and constituting means for neutralizing or balancing out the shunt capacitance disposed across said harmonic mode crystal body, a tuned tank circuit reactance means connected in shunt relation across said two adjacent arms of said bridge by means of a transformer, disposed in said feedback path and tuned substantially to said mechanical harmonic mode series-resonant frequency of said crystal body, said tank circuit reactance means being so disposed in circuit relation with respect to said feedback path as to constitute a discriminator reactance means for transmitting oscillations through said gain device at said desired harmonic mode frequency of said crystal body while simultaneously blockingor attenuating oscillations at the undesired fundamental mode frequency thereof, said discriminator reactance means comprising an inductance.

6. Harmonic crystal oscillator apparatus comprising an electronic gain device having oscillation generator electrodes comprising anode, control grid and grounded cathode electrodes, a bridge connected in circuit relation with said last-mentioned electrodes, a mechanical harmonic mode series-resonant frequency type piezoelectric crystal body disposed in one arm of said bridge in the feedback path of said gain device extending between said anode and grid electrodes thereof, a circuit including another arm in said bridge disposed adjacent said firstmentioned bridge arm, connected at its ends in circuit between said anode and grid electrodes of said gain device and constituting means for neutralizing or balancing out the shunt capacitance across said harmonic mode crystal body, a tuned-tank circuit reactance means connected in shunt relation across said two adjacent arms of said bridge, disposed in said feedback path and tuned substantially to said mechanical harmonic mode series-resonant frequency of said crystal body, and discriminator reactance means disposed in circuit relation with respect to said feedback path for transmitting oscillations through said gain device at said desired harmonic mode frequency of said crystal body while simultaneously blocking or attenuating oscillations at the undesired fundamental mode frequency thereof, said discriminator reactance means comprising capacitance means disposed in said feedback path in the part thereof extending between said bridge and said anode electrode circuit of said gain device.

7. Harmonic crystal oscillator apparatus comprising an electronic gain device having oscillation generator electrodes comprising plate, control grid and grounded cathode electrodes,a network system connected in circuit relation with said last-mentioned electrodes, a plurality of mechanical harmonic mode series-resonant frequency piezoelectric crystal bodies having different harmonic mode series-resonant frequencies spaced at intervals extending over a range or band of frequencies and adapted to be disposed in an arm of said network system in the feedback path of said gain device extending between said plate and grid electrodes thereof, means comprising switching apparatus disposed in said feedback path for in effect individually and selectively switching each of said crystal bodies in and out of circuit relation with, respect to said feedback path, a capacitative arm in said network system disposed adjacent said first-mentioned arm thereof and constituting means for neutralizing or balancing out the shunt capacitance disposed across said respective harmonic mode crystal bodies, a tuned tank circuit reactance means connected in shunt relation across said two adjacent network arms and tuned substantially to the mean frequency of the said mechanical harmonic mode frequencies of said crystal bodies, and discriminator reactance means disposed in circuit relation with respect to said feedback path for transmitting oscillations through said gain device at said desired harmonic mode frequency of each of said crystal bodies while simultaneously blocking oscillations at the undesired fundamental mode frequency of each of said crystal bodies.

8; Harmonic crystal oscillator apparatus comprising an electronic gain device having anode,

control grid and grounded cathode electrodes. a bridge connected in circuit relation with said last-mentioned electrodes, a harmonic mode series-resonant frequency type piezoelectric crystal body-disposed in one arm of said bridge'in the feedback path of said gain device extending between said anode and control grid electrodes thereof, capacitors disposed respectively in two other adjacent arms of said bridge and having a tap connection therebetween connected with said grounded cathode electrode of said gain device, a tank circuit disposedin that part of'said feedback path extending between said crystal body and said anode electrode of said gain device, tuned substantially to said harmonic mode series-resonant frequency of said crystal body oscillations through said gain device at said desired harmonic mode frequency thereof.

9. Harmonic crystal oscillator apparatus comprising a single pen'todetype electronic gain device having oscillation generator electrodes comprising plate, control grid and grounded cathode electrodes, a bridge connected in circuit relation with said last-mentioned electrodes, 2, mechanical harmonic mode series-resonant frequency type piezoelectric crystal body disposed in one arm of said bridge in the feedback path of said gain device extending between said plate and control grid electrodes thereof, a neutralizing capacitor disposed in another arm of said bridge, connected at its ends in circuit between said plate and control grid electrodes of said gain device and having a capacitance value sufficient to constitute means for neutralizing or balancing out the shunt capacitance across said harmonic mode crystal body, reactance devices disposed respectively in two other adjacent arms of said bridge andhaving a tap connection therebetween connected with said grounded cathode electrode of said gain device, a tuned tank circuit reactance device connected inshunt relation across said last-mentioned bridge arms and reactance devices, disposed in said feedback path extendin between saidplate and control grid electrodes of said gain device and tuned substantially to said mechanical harmonic mode series-resonant frequency of said crystal body, discriminator inductance means connected in shunt relation with respect to said bridge and with respect to said control grid and grounded cathode electrodes of said gain device, said inductance means having an inductance value sufficient for parallelresonating the circuit capacity disposed across said control grid and grounded cathode electrodes at a frequency value of such a relation to said desired harmonic mode frequency to thereby constitute means for preventing said crystal body from oscillating at its undesired fundamental mode frequency while simultaneously permitting oscillations therein at said desired harmonic mode frequency thereof.

10. Harmoniccrystal oscillator apparatus comprising a single pentode type electronic gain device having oscillation generator electrodes comprising plate, control grid and grounded cathode electrodes, a bridge connected in circuit relation with said last-mentioned electrodes, a mechanical harmonic mode series-resonant frequency type piezoelectric crystal body disposed in one arm of said bridge in the feedback path of said gain device extending between said plate and control grid electrodes thereof, a neutralizing capacitor disposed in another arm of said bridge, connected at its ends in circuit between said plate and control grid electrodes of said gain device and having a capacitance value suflicient to constitute means for neutralizing or balancin out the shunt capacitance across said harmonic mode crystal body, reactance devices disposed respectively in two other adjacent arms of said bridge and having a tap connection therebetween connected with said grounded cathode electrode of said gain device, a tuned tank circuit reactance device connected in shunt relation across said two last-mentioned bridge arms and reactance devices, disposed in said feedback path extending between said plate and control grid electrodes of said gain device and tuned substantially to said mechanical harmonic mode series-resonant frequency of said crystal body, discriminator inductance means connected in shunt relation with respect to said bridge and with respect to said control grid and grounded cathode electrodes of said gain device, said inductance means having an inductance value sufiicient for parallel-resonating the circuit capacity disposed across said control grid and grounded cathode electrodes at a frequency value of such a relation to said desired harmonic mode frequency to thereby constitute means for preventing said crystal body from oscillating at its undesired fundamental mode frequency while simultaneously permitting oscillations therein at said desired harmonic mode frequency thereof, and an output circuit for selecting electrical harmonies of the said desired crystal harmonic mode frequency oscillations comprising in the part of said feedback path extending between said bridge and said plate electrode of said gain device, a tuned inductance and capacitance system.

11. Harmonic crystal oscillator apparatus comprising a single pentode type electronic gain device having oscillation generator electrodes comprising plate, control grid and grounded cathode electrodes, a bridge connected in circuit relation with said last-mentioned electrodes, a mechanical harmonic mode series-resonant frequency type piezoelectric crystal body disposed.

in one arm of said bridge in the feedback path of said gain device extending between said plate and grid electrodes thereof, a neutralizing capacitor disposed in another arm of said bridge, connected at its ends in circuit between said plate and grid electrodes of said gain device and having a capacitance value sufficient to constitute means for neutralizing or balancing out the shunt capacitance across said harmonic mode crystal body, capacitors disposed respectively in two other adjacent arms of said bridge and having a tap connection therebetween connected with said grounded cathode electrode of said gain device, a tuned tank circuit inductance device connected in shunt relation across said two lastmentioned bridge arms and capacitors, disposed in said feedback path extending between said plate and control grid electrodes of said gain device and tuned substantially to said mechanical harmonic mode series-resonant frequency of said crystal body, discriminator inductance means connected in shunt relation with respect to said bridge and also said control grid and grounded cathode electrodes of said gain device, said inductance means having an inductance value suflicient for parallel-resonating the circuit capacity disposed across said control grid and grounded cathode electrodes of said gain device at a frequency value of such a relation to said desired harmonic mode frequency to thereby constitute means for preventing said crystal body from oscillating at its undesired fundamental mode frequency while simultaneously permitting oscillations therein at said desired harmonic mode frequency thereof, and an output circuit for selecting electrical harmonics of the said desired crystal harmonic mode frequency oscillations comprising in the part of said feedback path extending between said bridge and said plate electrode of said gain device a tuned inductance and capacitance system.

12. Harmonic crystal oscillator apparatus comprising a single pentode type electronic gain device having oscillation generator electrodes comprising plate, control grid and grounded cathode electrodes, a bridge connected in circuit relation with said last-mentioned electrodes, a plurality of mechanical harmonic mode seriesresonant frequency type piezoelectric crystal bodies having different harmonic mode seriesresonant frequencies spaced at intervals extending over a range or band of frequencies and adapted to be individually disposed in one arm of said bridge in the feedback path of said gain device extending between said plate and control grid electrodes thereof, means comprising a plurality of switching devices for in effect individually and selectively switching each of said harmonic mode crystal bodies in and out of circuit relation with respect to said feedback path, a neutralizing capacitor disposed in another arm of said bridge, connected at its ends in circuit between said plate and control grid electrodes of said gain device and having a capacitance value sufficient to constitute means for neutralizing or balancing out the shunt capacitance across each of said harmonic mode crystal bodies, capacitors disposed respectively in two other adjacent arms of said bridge and having a tap connection therebetween connected with said grounded cathode electrode of said gain device, a tuned tank circuit inductance device connected in shunt relation across said two last-mentioned bridge arms and capacitors, disposed in said feedback path extending between said plate and grid 4 electrodes of said gain device and tuned substantially to the mean frequency of the said mechanical harmonic mode series-resonant frequencies of said crystal bodies, discriminator inductance means connected in shunt relation across said bridge and across said control grid and grounded cathode electrodes of said gain device, said inductance means having an inductance value sufficient for parallel-resonating the circuit capacity disposed across said grid and grounded cathode electrodes of said gain device at a frequency value of such a relation to each of said desired harmonic mode frequencies of said crystal bodies to thereby constitute means for preventing each of said crystal bodies from oscillating at its undesired fundamental mode frequency while permitting oscillations therein at its desired mechanical harmonic mode frequency, and an output circuit for selecting electrical harmonics of said desired harmonic mode frequency oscillations of said crystal bodies comprising a tuned inductance and capacitance systern connected with the part of said feedback pathextending between said bridge and said plate electrode of said gain device.

13. Harmonic crystal oscillator apparatus comprising a single pentode type electronic gain device having oscillation generator electrodes comprising plate, control grid and grounded cathode electrodes, a bridge connected in circuit relation with said last-mentioned electrodes, a plurality of mechanical harmonic mode series-resonant frequency type piezoelectric crystal bodies having different harmonic mode series-resonant frequencies spaced at intervals extending over a range or band of frequencies and adapted to be individually disposed in one arm of said bridge in the feedback path of said gain device extending between said plate and control grid electrodes thereof, means comprising a plurality of switching devices for in effect individually and selectively switching each of said harmonic mode crystal bodies in and out of circuit relation with respect to said feedback path, a neutralizing capacitor disposed in another arm of said bridge, connected at its ends in circuit between said plate and control grid electrodes of said gain device and having a capacitance value sufficient to constitute means for neutralizing or balancing out the shunt capacitance across each of said harmonic mode crystal bodies, capacitors disposed respectively in two other adjacent arms of said bridge and having a tap connection therebetween connected with said grounded cathode electrode of said gain device, a tuned tank circuit inductance device connected in shunt relation across said two last-mentioned bridge arms and capacitors, disposed in said feedback path extending between said plate and grid electrodes of said gain device and tuned substantially to the mean of the said mechanical harmonic mode series-resonant frequencies of said crystal bodies, discriminator inductance means connected in shunt relation across said bridge and across said control grid and grounded cathode electrodes of said gain device, said inductance means having an inductance value sufficient for parallel-resonating the circuit capacity disposed across said grid and grounded cathode electrodes of said gain device at a frequency value of such relation to each of said desired harmonic mode frequencies of said crystal bodies to thereby constitute means for preventing each of said crystal bodies from oscillating at its undesired fundamental mode frequency while permitting oscillations therein at its desired mechanical harmonic mode frequency, and an output circuit for selecting electrical harmonics of said desired harmonic mode frequency oscillations of said crystal bodies comprising a tuned inductance and capacitance system connected with the part of said feedback path extending between said bridge and said plate electrode of said gain device, said switching devices comprising rectifiers individually disposed in series circuit relation with said respective harmonic mode crystal bodies.

14. Harmonic crystal oscillator apparatus comprising an electronic device having plate, control gridand grounded cathode electrodes, a bridge connected in circuit relation with said last-mentioned electrodes, a harmonic mode series-resonant frequency type piezoelectric crystal body disposed in one arm of said bridge, resistors disposed in two other adjacent arms of said bridge and having a tap connection therebetween connected with said grounded cathode electrode of said gain device, means comprising a tuned circult inductance element disposed in said grid electrode circuit in series circuit relation with said harmonic mode crystal body for varying or shifting said oscillator frequency, said tuned circuit inductance element in combination with the oscillator circuit capacitance as disposed in shunt relation across said grid and grounded cathode electrodes of said gain device constituting means for producing a substantially 90 degrees phase shift in said grid electrode circuit, and means for producing an additional substantially 90 degrees phase shift at the input-terminals of said bridge comprising a relatively high leakage inductance means coupling said resistors in said bridge arms to said plate output circuit of said gain device, said pair of substantially 90 degrees phase shift means constituting means for-producing a substantially 180 degrees phase shift in the feedback path of said gain device extending between said plate output and grid electrodes thereof.

15. Harmonic crystal oscillator apparatus comprising a single electronic gain device having plate, control grid and grounded cathode electrodes, a networkconnected in circuit relation with said last-mentioned electrodes, a harmonic mode series-resonant type piezoelectric crystal body disposed in one arm of said network in the feedbackpath of said gain device extending from said plate to said grid electrodes thereof, a resistor disposed in another arm of said network in circuit between one end of said crystal body and said grounded cathode electrode of said gain device, an impedance limiting'resistor disposed across said network in circuit between the other end of said crystai body and said grounded cathode electrode of said gain device, said crystal body being operatively disposed between said network arm resistor and saidimpedance limiting resistor and said last-mentioned resistors having low values of impedance relative to circuit capacitive reactances shunting them, means for obtaining a substantially degrees phase shift in said grid electrode circuit comprising a tuned circuit, said tuned circuit comprising an inductance element disposed in series relation in said grid electrode circuit between said grid electrode and the grid end of said impedance limiting resistor and said tuned circuit also comprising the capacitance of said oscillator circuit as measured between said grid and grounded cathode electrodes of said gain device, and means for obtaining an additional substantially 90 degrees phase shift in said plate electrode circuit coupling said plate electrode to said network comprising capacitance means disposed in series circuit relation in said plate electrode and network coupling circuit.

16. Harmonic crystal oscillator apparatus comprising a single pentode type electronic gain device having oscillation generator electrodes comprising output or plate, input or control grid and grounded cathode electrodes, a bridge connected in circuit relation with said last-mentioned electrodes, a mechanical harmonic mode series-reso nant frequency type piezoelectric crystal body disposed in one arm of said bridge in the feedback path of said gain device extending between said plate and grid electrodes thereof, a neutralizing capacitor disposed in another arm of said bridge, connected at its opposite ends in circuit between said plate and grid electrodes of said gain device and having a capacitance value sufficient to constitute means for neutralizing or balancing out the shunt capacitance across said harmonic mode crystal body, impedance devices disposed respectively in two other adjacent arms 29 of said bridge and having a tap connection therebetween connected with said grounded cathode electrode of said gain device, a tuned tank circuit reactance device operatively connected in shunt relation across said two last-mentioned bridge arms, disposed in said feedback path extending between said plate and grid electrodes of said gain device and tuned substantially to said mechanical harmonic mode series-resonant frequency of said crystal body, means comprising a discriminator reactance circuit connected in circuit relation with said feedback path and across said bridge for attenuating oscillations at the undesired fundamental mode frequency of said crystal body while simultaneously permitting oscillations therein at said desired harmonic mode frequency thereof and a plate circuit output for said desired harmonic mode frequency oscillations.

17. Harmonic crystal oscillator apparatus comprising a single pentode type electronic gain device having oscillation generator electrodes com prising output or plate, input or control grid and grounded cathode electrodes, a bridge connected in circuit relation with said last-mentioned electrodes, a mechanical harmonic mode seriesresonant frequency type piezoelectric crystal body disposed in one arm of said bridge in the feedback path of said gain device extending between said plate and grid electrodes thereof, a neutralizing capacitor disposed in another arm of said bridge, connected at its opposite ends in circuit between said plate and grid electrodes of said gain device and having a capacitance value fillmclent to constitute means for neutralizing or 30 balancing out the shunt capacitance across said harmonic mode crystal body, impedance devices disposed respectively in two other adjacent arms of said bridge and having a tap connection therebetween connected with said grounded cathode electrode of said gain device, a tuned tank circuit reactance device operatively connected in shunt relation across said two last-mentioned bridge arms, disposed in said feedback path extending between said plate and grid electrodes of said gain device and tuned substantially to said mechanical harmonic mode series-resonant frequency of said crystal body, means comprising a discriminator reactance circuit connected in shunt relation across said bridge for attenuating oscillations at the undesired fundamental mode frequency of said crystal body While simultaneously permitting oscillations therein at said desired harmonic mode frequency thereof, a plate circuit output for said desired harmonic mode frequency oscillations, and means comprising a reactance device connected in series circuit relation in that part of said feedback path between said bridge and said input grid electrode of said gain device for adjusting the frequency of said desired harmonic mode frequency oscillations.

BURTON H. SIMONS.

REFERENCES CITED UNITED STATES PATENTS Name Date F ir O t- 2 19 Number 

